Experimental evidence of sixfold oxygen coordination for phosphorus

Phosphorus, a group V element, has always been found so far in minerals, biological systems, and synthetic compounds with an oxygen coordination number of four (i.e., P[O.sub.4] groups). We demonstrate using phosphorus K-edge XANES spectroscopy that this element can also adopt a sixfold oxygen coordination (i.e., P[O.sub.6] groups). This new coordination was achieved in phosphorus-doped (1 wt% [P.sub.2] [O.sub.5] level) Si[O.sub.2]-stishovite synthesized at 18 GPa and 1873 K and quenched to ambient conditions. This change of phosphorus coordination at high pressure within a dense silicate structure is particularly relevant to phosphorus mineralogy (and geochemistry) in the deep Earth. With a mantle abundance below 0.25 wt%, phosphorus has been shown to be mainly hosted by silicates (e.g., olivine) in the Earth's upper-mantle, in the fourfold-coordinated silicon sites. In the lower mantle where all silicon is sixfold coordinated, we show here that phosphorus has the crystal-chemical ability to remain incorporated into silicate structures. Keywords: Phosphorus, coordination, XANES, stishovite, berlinite, lower mantle